A specific type of steel sheets which are referred to as “DR (double reduced) steel sheets” may be used in the production of lids and bottoms of beverage cans and food cans, bodies of three-piece cans, drawn cans, and the like. DR steel sheets are produced by performing cold rolling, annealing, and again cold rolling. The thickness of DR steel sheets can be readily reduced compared with SR (single reduced) steel sheets, which are produced by performing only temper rolling subsequent to the cold-rolling and annealing steps.
One of the ways to reduce the cost of producing cans is to reduce the weights of members constituting the cans. For example, it is possible to reduce the weights of can lids by reducing, for example, the thickness of a material of the can lids. Thus, reducing the thickness of a steel sheet used in the production of can lids by using DR sheets or the like makes it possible to reduce the cost of producing cans.
While reducing the thickness of a steel sheet used in the production of can lids and the like makes it possible to reduce the cost of producing cans, it is necessary to prevent the strength of the can lids and the like from decreasing. Thus, it is necessary not only to reduce the thickness of the steel sheet but also to increase the strength of the steel sheet. For example, in the case where thin DR sheets are used, the DR sheets are required to have a tensile strength of about 400 MPa or more in order to produce cans having a certain strength. However, high-strength steel sheets having a smaller thickness than steel sheets that have been used in the related art are likely not capable of withstanding works. Specifically, a can is produced by performing blanking, a shell forming, and a curl forming (curling) in this order by press forming in order to form a lid, and subsequently seaming the flange portion of a can body with the curled portion of the lid in order to seal the can. In the curl forming, which is performed in the periphery of the lid, is likely to cause wrinkling to occur. Therefore, thin high-strength sheets have low formability despite their sufficiently high strength.
In the case where lids are produced from thin, high-strength sheets, buckling may occur in the circumferential direction when a diameter-reduction work is performed as a curl forming in order to reduce the diameter of the lid to be smaller than the diameter of the blank. In order to reduce the occurrence of buckling, in some cases, the curl forming is performed using, for example, inner and outer molds. However, introducing a new curl-work facility requires a large amount of capital investment.
In the production of DR sheets, cold rolling is performed subsequent to annealing. This causes work hardening. Thus, DR sheets are thin, hard steel sheets. DR sheets have poorer ductility and poorer workability than SR sheets. Therefore, in most cases, using the DR sheets requires the improvement of the workability of the DR sheets.
In addition to sanitary ends, there has been a widespread use of EOE (easy open end) cans that can be opened without a can opener. In the production of EOE cans, it is necessary to form a rivet, to which a tab is attached, by bulging and drawing. This work requires a certain degree of ductility of a material which corresponds to an elongation of about 10% in a tensile test.
Although it is difficult to achieve the certain degrees of ductility and strength described above by using DR sheets that have been used in the related art, there has been a growing demand for the application of DR sheets to the production of EOE cans and beverage cans from the viewpoint of a reduction in the cost of producing cans.
Patent Literature 1 discloses a technique in which the solute N content (Ntotal-NasAlN) in a steel sheet containing, by mass, C: 0.02% to 0.06%, Si: 0.03% or less, Mn: 0.05% to 0.5%, P: 0.02% or less, S: 0.02% or less, Al: 0.02% to 0.10%, N: 0.008% to 0.015%, and the balance being Fe and inevitable impurities is limited to be 0.006% or more, the total elongation of the steel sheet subjected to an aging treatment is limited to be 10% or more in the rolling direction and 5% or more in the width direction, and the average Lankford value of the steel sheet subjected to the aging treatment is limited to be 1.0 or less.
Patent Literature 2 discloses a technique in which the solute N content in a steel sheet containing, by mass, C: more than 0.02% and 0.10% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.20% or less, Al: 0.10% or less, N: 0.0120% to 0.0250%, solute N: 0.0100% or more, and the balance being Fe and inevitable impurities is limited to be a predetermined value or more, and the steel sheet is hardened by quench aging and strain aging performed in a printing step, a film-laminating step, a drying-baking step, or the like that are conducted before the steel sheet is formed into cans in order to increase the strength of the steel sheet. Patent Literature 2 also discloses a method for producing a steel sheet in which hot rolling is performed such that the slab-extraction temperature is 1200° C. or more and the finishing-rolling temperature is (Ar3 transformation temperature—30°) C. or more and the resulting hot-rolled sheet is coiled at 650° C. or less.